Heat the milk with the cream slightly. Separately, beat the egg, 2 of the yolks, the sugar and starch to a homogeneous mixture.
Add 1/3 of the milk-cream mixture and beat. Boil the rest of the milk mixture. Add the vanilla to it. Once boiling, pour the egg mixture in a thin trickle while stirring nonstop until thickened.
Leave it aside to cool. The cream needs to be cold when putting it in the puff pastries. Roll out the puff pastry into a sheet about 1/4″ (0.5 cm) thick. Cut it into squares.
Put 1 tsp of the cream mixture in the middle of each, then fold into a triangle. Smear with egg yolk and bake at 392°F (200 °C) until reddened. Optionally, sprinkle them with powdered sugar.